The present invention relates to a method for the preparation of a polyvinyl chloride resin or, more particularly, to a method for the preparation of a polyvinyl chloride resin by the process of suspension polymerization of vinyl chloride monomer in an aqueous polymerization medium containing a water-soluble dispersing agent dissolved therein capable of giving a resin product containing a greatly decreased amount of fish eyes and having a greatly increased porosity of the resin particles.
As is well known, polyvinyl chloride, referred to as PVC hereinbelow, resins are mostly produced in the form of a resin powder by the method of suspension polymerization of vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride in an aqueous polymerization medium containing a water-soluble dispersing agent dissolved therein to stabilize the suspension of the monomer droplets in the aqueous medium. Among the various requirements for the PVC resin products, higher porosity of the resin particles and smaller content of fish eyes are some of the most important properties of the resin powder from the standpoint of workability. In this regard, a proposal has been made in Japanese Patent Kokai 52-5886, 52-15780, 55-112210 and 53-6392 and Japanese Patent Publication 61-18562, according to which the water-soluble dispersing agent is used in combination with a partially saponified polyvinyl alcohol insoluble in water but soluble in the monomer having a low degree of saponification not exceeding 55% and a low average degree of polymerization not exceeding 600. This method is indeed effective in obtaining a PVC resin having somewhat increased porosity and somewhat decreased content of fish eyes as well as a decreased content of unreacted vinyl chloride monomer absorbed in the resin particles which may cause a serious problem of environmental pollution. This method is not quite satisfactory in all respects because, when the plasticizer used in compounding a resin composition is a trimellitate ester-based or polyester-based one, the amount of the fish eyes is rather increased and the increase in the porosity of the particles causes an increase in the apparent consistency of the polymerization mixture in the form of an aqueous slurry, especially, in the later stage of the polymerization reaction so that the polymerizate slurry is less flowable to decrease the efficiency for the removal of the heat of polymerization resulting in an eventual degradation in the quality of the resin product.